In sheet-fed offset printing machines, the supply of printing ink is generally accomplished by means of an undershot inking unit. Undershot inking units comprise an ink fountain and associated metering devices, such as ink-metering elements or undivided ink doctor blades; an ink fountain roller; an intermittent ductor roller, and one or more inking rollers. By means of the metering elements, the ink layer thickness on the ink fountain roller is adjusted in accordance with the requirements of the printing machine. The intermittent ductor roller, as a result of periodic contact with the ink fountain roller, removes a strip of ink of a certain length from the ink fountain roller and transfers the ink onto the first inking roller. This first inking roller is usually designed as an axially reciprocating distributor roller for contacting further inking rollers through traversing movements of adjustable stroke and/or frequency. By means of these further inking rollers, the ink quantity fed by the ductor roller splits and correspondingly leads to an inking of the printing regions on the printing form or plate located on the plate cylinder.
Typically, the intermittent ductor roller is rotatably journalled in a pair of pivotable bearing levers. Each of the pivotable bearing levers is coupled with a cam follower roller drive, by which an intermittent or pendulating movement of the ductor roller between the ink fountain roller and the distributor roller is achieved. The cam disk of the intermittent ductor roller drive usually is driven directly from the printing mechanism, with a corresponding reduction in rotational speed (such as a 3:1 reduction). Thus, a ductor stroke, or pendulating movement of the ductor roller between the ink fountain roller and the inking roller, occurs with respect to a corresponding number of revolutions of the plate cylinder (such as 3).
The ink fountain roller can have a mechanical drive derived from the printing mechanism, or it may have a controllable electrical drive. In conventional undershot inking units, the rotational speed of the ink fountain roller is dependent upon the printing speed of the printing machine, and may vary continuously or incrementally with the printing speed. Such conventional ink fountain rollers are described in U.S. Pat. No. 4,007,683 and in EP 518,234 A1 and EP 264,838 B1.
The amount of ink fed to the inking roller in such conventional devices is controlled by regulating the rotational contact angle, or contact time, of the ductor roller on the ink fountain roller so as to regulate the ductor-strip width. Regulation of the ductor-strip width is accomplished either by adjusting the design of the cam disk for the ductor roller drive by means of adjustable control planes, or by modulating the speed of the ink fountain roller. Where the ductor-strip width is regulated by modulation of the ink fountain roller, the rotational speed of the ink fountain roller is determined by the speed of the printing machine and by independent adjustment. Thus, the characteristic curve by which the rotational speed of the ink fountain roller is coupled with the printing machine speed is varied. Where the ductor-strip width is adjusted by means of a variable cam disk, the coupling of the rotational speed of the ink fountain roller with the speed of the printing machine is obtained by an invariable characteristic curve. EP 264,838 B1 discloses the regulation of the ductor-strip width by regulating the ink fountain roller speed by means of a switching drive, which controls the step width of the ink fountain roller. The drive of the ductor roller and the corresponding switching drive for the ink fountain roller operate in phase in such manner that the rotation of the ink fountain roller occurs in the phase of contact with the ductor roller.
The distribution of ink is typically determined during setup of the printing machine. This generally occurs at a relatively low printing speed, such as 5,000 sheets per hour with a conventional offset printing machine. During actual production, however, the production speeds of such machines can be greater than 15,000 sheets per hour. When the undershot inking unit has been calibrated at a lower printing speed, it is invariably observed that the ink densities detected on a print check strip and in the image decrease as a whole. The ink density changes from low to high production speed are typically quite significant in the prior art machines. This effect is know as ink fall-off.
Conventional inking mechanisms have thus far been unable to resolve the problem of ink fall-off. The present invention seeks to provide an apparatus and associated method for overcoming the problems associated with conventional inking mechanisms in this regard.